1. Field of the Invention
This invention relates to a process and accompanying apparatus for the liquefaction of gaseous hydrocarbons, such as natural gas, by an improved single mixed refrigerant process, and more particularly relates to such a process, the capacity of which is easily and economically adaptable to changes in environmental ambient temperature while requiring only insignificant additional power requirements.
2. Description of the Prior Art
Since the discovery of the cryogenic gas liquefaction process by Karl Von Linde in 1895, a variety of such processes have been developed for the liquefaction of hydrocarbons, particularly for hydrocarbons such as natural gas. Such processes are frequently carried out at the site of the gas recovery.
Natural gas is being utilized as a fuel in ever increasing amounts in recent years. As the population of large metropolitan areas increases, the utilization of natural gas as a heating medium for these large cities has become, like other power sources, extremely volatile due to the particular seasonal demand. Natural gas is utilized primarily during winter for domestic heating and the like, and its use naturally falls off drastically during the warm summer months. In an attempt to smooth out such natural gas demand fluctations, a number of LNG (liquid natural gas) peak shaving plants are now being employed. Such systems typically liquefy the natural gas between the months of April and October, storing the LNG in enormous, flat-bottom storage tanks, typically kept by necessity at around atmospheric pressure, and then during peak demand season in the cold weather, vaporize and utilize the LNG.
Since these peak shaving plants are typically operated only about 200-220 days a year and have a much smaller capacity as compared to the large, so-called "base-load" plants present at the source of gas production, economic factors involving both their construction and ease of operation are even more important than usual. Consequently, the classic cascade process, which utilizes nine separators and three compressors, is simply too expensive for utilization as a peak shaving unit. In 1959, A. P. Klemenko (Refrigeration Congress, 959, Copenhagen) disclosed a process for employing a mixture of refrigerants in conjunction with a single compressor and a simplified cascade process. A number of mixed refrigerant processes have also been developed by the art, such as U.S. Pat. Nos. 3,218,816, 3,581,511 and U.S. Pat. No. 3,364,685.
U.S. Pat. No. 4,033,735 discloses the so-called Prico Process, involving a single stage compression operation in which the resulting vapor and liquid streams which leave the aftercooler separator are combined outside the cold box before entering into the warm end heat exchanger. Also, the process does not provide for a separation of a refrigerant mixture during the entire heat exchange with the natural gas feedgas, and thus cannot be considered a cascade cycle.
Recently, the art has begun to utilize a single mixed refrigerant cycle containing two refrigerant separators for the simplified cascade process, one such separator utilized for the aftercooler separator and one for the cold box separator. This process has become popular because it is more efficient than the Prico Process of U.S. Pat. No. 4,033,735, for it is able to more efficiently match the particular refrigerant mixtures to the required temperature levels. A number of processes which use a single mixed refrigerant system, typically 4-6 refrigerant components, are described in, for example, LNG Information Book, July 1968, Amer. Gas Assoc., Inc., New York, N.Y.; "Modified Cascade Cycle", pp. 25-27 (July, 1968), or, Linde Reports on Science & Technol. (Linde, A. G., Vol. 32, (1981), p. 19-28, "Liquefaction of Natural Gas with Aid of Refrigerant Mixtures", Hans-Robert Zollner). These processes are more efficient than the other simplified processes such as the N.sub.2 /CH.sub.4 expander cycle, because due to the absence of an expander they are able to condense part of the refrigerant mixture, utilizing ambient air or cooling water as the cooling fluid. However, these systems are hampered by a problem not common to the other refrigeration processes, that is, the temperature of the aftercooler separator must be kept constant once it is designed, regardless of the particular environmental ambient temperature of the cooling fluid utilized by the aftercooler. If the percentage of refrigerant which is condensed in the compressor aftercooler should vary from the process design, one of the heat exchanger units in the cold box will be unable to carry out its refrigeration duty. This is because, in the conventional process, the vapor fraction from the aftercooler separator has been designed to supply the refrigeration duty of the cold end heat exchanger and the liquid fraction from the aftercooler separator is utilized to supply the refrigeration of the warm end exchanger. Consequently, when the ambient temperature falls, since the percentage amounts of liquid and vapor separated in the aftercooler separator remain constant, and the unit remains at the same temperature, only very small benefits are derived from this change. In such systems, a significant amount of additional condensation of the refrigerant mixture does not occur in the aftercooler unit. Thus, modifying the two separator-three heat exchanger-mixed refrigerant process, to enable an improved flexibility in adapting to the temperature of the ambient air, is both difficult and expensive.
The Prico Process is more adaptable than the aforementioned conventional single mixed refrigerant process which utilizes multiple separators, since it does not utilize the vapor and liquid fractions leaving the aftercooler separator in a separate fashion. However, the process utilizes only 20-40.degree. F. of the superheating of the refrigerant which leaves the cold box, and thus is only able to obtain a maximum liquefaction capacity increase of about 5% due to the lowering of the ambient temperature.
As a result, the art has been forced to design LNG peak shaving plants that employ as the design ambient temperature the highest temperature usually occurring during the summer season of the particular local, which is frequently about 20-30.degree. F. above the average ambient temperature over the 200 day operating season. Consequently, such a design not only requires a larger compressor, but also results in high operating costs.